1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting and receiving multi-user control channels for a plurality of user equipment in a wireless communication system using multiple antennas and, more particularly, to a method and apparatus for transmitting and receiving control channels multiplexed being based on a space division in a data channel region and also a definition of a control channel search space in consideration of characteristics of the data channel region.
2. Description of the Related Art
Modern mobile communication systems are evolving toward high-speed, high-quality wireless packet data communication systems for providing data services and multimedia services as well as offering traditional voice-based services. In order to support a high-speed, high-quality wireless packet data transmission service, a variety of mobile communication standards, such as HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access) of 3GPP (the third Generation Partnership Project), HRPD (High Rate Packet Data) of 3GPP2, IEEE 802.16, and the like, have been developed in the art.
The existing third-generation wireless packet data communication systems such as HSDPA, HSUPA, HRPD, etc. use the Adaptive Modulation and Coding (AMC) technique, the channel dependent scheduling technique, etc. in order to improve its transmission efficiency. The AMC technique allows a transmitter to adjust the amount of transmitted data according to channel conditions. Namely, the transmitter decreases the amount of transmitted data under unfavorable channel conditions and thereby sets the probability of errors in receiving data to a desired level. Also, the transmitter increases the amount of transmitted data under favorable channel conditions and thereby effectively transmits a lot of information while setting the probability of errors in receiving data to a desired level. Meanwhile, the use of the channel dependent scheduling technique allows a transmitter to provide service to selected user equipment with excellent channel conditions among several user equipment. Therefore, system capacity is increased more in this case than in other cases wherein a channel is allocated to only one of the user equipment. Normally this increase of capacity refers to a multi-user diversity gain. In short, the AMC technique and the channel dependent scheduling technique may apply a proper modulation and coding type at the most efficient time determined depending on the feedback of information about partial channel conditions received from a receiver.
A recent trend in the art is to replace the CDMA (Code Division Multiple Access) method used in the second or third generation mobile communication systems with the OFDMA (Orthogonal Frequency Division Multiple Access) method in the next generation system. Currently, the 3GPP and the 3GPP2 are performing standardization for evolved systems using the OFDMA method, which is expected to produce a capacity increase in comparison with CDMA. One reason for a capacity increase in OFDMA is to allow frequency domain scheduling. As the channel dependent scheduling technique based on time-dependent variations of a channel produces a capacity gain, so can using frequency-dependent variations of a channel.
When the AMC technique and the channel dependent scheduling technique are realized, a base station adaptively allocates given wireless resources such as frequency, time, power, etc. according to the channel condition of user equipment. This adaptive allocation information is sent from a base station to user equipment through PDCCH (Physical Downlink Control Channel). By receiving PDCCH, the respective user equipment recognize their allocated wireless resources.
The allocation of wireless resources includes resource allocation for the downlink from a base station to user equipment and resource allocation for the uplink from user equipment to a base station. Downlink resource allocation is adaptively made according to channel condition reported by user equipment as well as the amount of data to be transmitted from a base station to the user equipment. A base station reports, through PDCCH, which resource is allocated to which user equipment for data transmission and which transport format indicating a modulation and coding type is used. Through PDCCH information, each user equipment recognizes whether a downlink resource is allocated and, if so, how to receive the transmitted signal through the allocated resource. Similarly, uplink resource allocation is adaptively made according to channel condition reported by user equipment as well as the amount of data to be transmitted. A base station reports, through PDCCH, which resource is allocated to which user equipment for data transmission and which transport format is used to send data to the allocated resource. Through PDCCH information, each user equipment recognizes whether an uplink resource is allocated and, if so, which transport format should be used.
Downlink Control Information (DCI) contained in PDCCH for a downlink resource allocation is generally as follows.
User Equipment Identification (UE ID) refers to information used for user equipment to determine whether there exists a signal transmitted to user equipment. Normally CRC (Cyclic Redundancy Check) depending on specific UE ID is inserted into DCI, so if specific user equipment successfully restores DCI, such control information is considered as information for such user equipment.
If DCI is successfully restored, user equipment recognizes from the Downlink Resource Block (DL RB) allocation information which resource blocks its own data is actually sent through.
Transport Format (TF) refers to a modulation and coding type of a transmitted signal. If the AMC technique is used, user equipment should know TF in order to perform a demodulation and decoding process.
Hybrid Automatic Repeat reQuest (HARQ) refers to a process in which a receiver informs a transmitter whether a transmitted packet is successfully received and then, in case of success, the transmitter sends the next packet and in case of failure, resends the same packet. HARQ related information includes a HARQ process number and any other related information such as an indication that a certain transmitted signal is original or repeated. Based on HARQ related information, user equipment determines whether to decode a currently received packet by combining with an earlier received packet or to decode it newly.
Additionally, DCI contained in PDCCH may further include information for transmission through multiple antennas, information for a power control, information about whether distributed transmission is used or not, and the like.
On the other hand, information contained in PDCCH for an uplink resource allocation is generally as follows.
If control information is successfully restored, user equipment recognizes from UL RB allocation information which resource blocks the data should be sent through.
User equipment should know the Transport Format (TF) to be used in order to create a signal in a demodulation and decoding manner requested by a base station.
Additionally, DCI may further include information about an uplink reference signal for supporting multiple access to the uplink space domain, information about whether distributed transmission is used or not, information about whether a channel condition report is requested or not, and the like.
FIG. 1 is a diagram illustrating a method for setting a control channel candidate group in a conventional wireless communication system.
Referring to FIG. 1, a Control Channel Element (CCE) refers to a unit of a logical channel that forms PDCCH. Particularly, a one-to-one correspondence exists between CCEs and Resource Elements (REs) which are units of a physical channel. Meanwhile, an Aggregation Level (AL) indicates how many CCEs constitute PDCCH. Namely, if PDCCH consists of N pieces of CCEs, AL becomes N. FIG. 1 shows examples of AL 1 in reference numeral 111, AL 2 in reference numeral 112, AL 4 in reference numeral 113 and AL 8 in reference numeral 114. If PDCCH uses one modulation type, the number of encoded bits to be transmitted reduces as AL decreases. This means that the code rate of channels in PDCCH is reduced. Namely, in case of a low AL, control information can be sent through fewer resources. However, user equipment can successfully receive it under a good channel condition. In case of a high AL, while relatively more resources are used, user equipment can successfully receive control information even under poor channel condition. For an effective use of resources, it is desirable that a control channel may be formed with a low AL for user equipment having good channel condition but formed with a high AL for user equipment having poor channel condition.
Additionally, the number of information bits forming DCI may be varied according to the attribute of control information. For instance, resource block allocation information may use many bits in order to increase the degree of freedom or use fewer bits instead of reducing the degree of freedom. Also, the number of information bits forming DCI is varied depending on whether to include various types of additional information. When different numbers of bits may form different DCIs, they are distinguished from each other by DCI format. Since user equipment does not know DCI format with which PDCCH is transmitted, it will be blind-decoded. Although transmitted to user equipment having the same channel conditions, PDCCH with DCI format using many bits, may be preferably transmitted with a higher AL than PDCCH with DCI format using few bits.
Specifically, let's suppose that eight CCEs, designated by reference numerals 100 to 107, are given. The number of CCEs is only exemplary and may be varied frequently. Factors that affect the number of CCEs include static values such as a downlink system bandwidth, the number of transmitting antennas of a base station, and the number of downlink ACK/NACK channels for supporting uplink HARQ, and control region information the value of which is changed at every sub-frame that is a time unit for scheduling.
Reference numerals 120 to 127 are PDCCH candidates for AL 1 111. For instance, reference numeral 120 indicates that PDCCH is formed using the only CCE 0 100, and a reference numeral 127 indicates that PDCCH is formed using the only CCE 7 107. Reference numerals 128 and 129 are PDCCH candidates for AL 2 112. For instance, reference numeral 128 indicates that PDCCH is formed using CCE 0 100 and CCE 1 101. Reference numeral 132 is one of PDCCH candidates for AL 4 113 and indicates that PDCCH is formed using four CCEs from CCE 0 100 to CCE 3 103. Reference numeral 134 is the only PDCCH candidate for AL 8 114 and indicates that PDCCH is formed using eight CCEs from CCE 0 100 to CCE 7 107.
A way of forming PDCCH candidates according to AL is based on a tree structure. Namely, AL 2 112 consists of a set of PDCCH candidates with AL 1 111, and AL 4 113 consists of a set of PDCCH candidates with AL 2 112. Also, AL 8 114 consists of a set of PDCCH candidates with AL 4 113. For instance, PDCCH indicated by reference numeral 132 is formed of four CCEs from CCE 0 to CCE 3, and this is a combination of a PDCCH candidate 128 formed of CCE 0 and CCE 1 and a PDCCH candidate 129 formed of CCE 2 and CCE 3 in AL 2 112. According to such a tree structure, when the total number of CCEs is N_CCE, the number of possible PDCCHs in specific AL is calculated by floor(N_CCE/AL). Here, floor(x) means a round-down function, which results in the maximum integer smaller than or equal to x.
The user equipment should attempt to perform a blind decoding in a PDCCH candidate group in order to find a downlink control channel transmitted to itself among several downlink control channels. Here, a blind decoding means that, when a base station sends PDCCH through one of control channel candidates defined in a control channel candidate group, user equipment receives PDCCH without any information about a control channel candidate through which control channel information is sent. For an effective blind decoding, a conventional Orthogonal Frequency Division Multiplexing (OFDM)-based LTE (Long Term Evolution) system defines a search space of PDCCH candidates for each user equipment as:The first CCE location of the m-th PDCCH candidate for a given AL=AL{(Yk+m) mod floor(N_CCEk/AL)}, m=0, . . . , MAL−1  Equation (1)
Here, N_CCEk refers to the total number of CCEs in the k-th sub-frame, and MAL refers to the number of PDCCH candidates in a PDCCH candidate group at each AL. Also, Yk is equal to (39827×Yk-1) mod 65537, and Y−1 indicates UE ID. Additionally, x mod y is a function that indicates the remainder after x is divided by y. If the first CCE location for a given AL is known from Equation (1), PDCCH candidates for a given AL are AL pieces of CCEs from the first CCE location.
FIG. 2 is a diagram illustrating the structure of a downlink sub-frame in a conventional LTE system.
Referring to FIG. 2, a single sub-frame 215 is composed of fourteen OFDM symbols designated by reference numerals 200 to 213. The front three symbols 200, 201 and 202 constitute a PDCCH region allocated to control channels (i.e., PDCCH), and the rest, 203 through 213, is a PDSCH (Physical Downlink Shared Channel) region allocated to data channels (i.e., PDSCH). While PDCCH is transmitted through the entire system bandwidth in the PDCCH region 200 to 202, PDSCH is transmitted on the basis of a Resource Block (RB) 214 that is a basic unit of scheduling. Each RB consists of twelve sub-carriers, and the total number of RBs is varied according to a system bandwidth. The reason that the PDCCH region 200 through 202 is located at the front of the sub-frame 215 is to allow user equipment to check first PDCCH. If PDCCH has no pertinent data, user equipment enters into a micro sleep mode in order to reduce power consumption in the PDSCH region 203 through 213.
FIG. 3 is a diagram illustrating the structure of downlink resource blocks in a conventional LTE-Advanced system.
Referring to FIG. 3, a common reference signal 300 in a PDCCH region 304 is used for channel estimation for PDCCH decoding, and a common reference signal 300 in a PDSCH region 305 is used for downlink channel measurement. Additionally, channel estimation for data decoding in the PDSCH region 305 uses code division multiplexing reference signal groups 301 and 302. Each of these groups 301 and 302 is multiplexed into a reference signal for multiple layers by using an orthogonal sign. For instance, in case of transmission of four layers, an orthogonal sign with a length 2 is applied to two reference signal REs that are continuous on a time domain, so two different reference signals are multiplexed for each reference signal group. Similarly, in case of transmission of eight layers, an orthogonal sign with length 4 is applied to four reference signal REs that are spread on a time domain, so four different reference signals are multiplexed for each reference signal group.
In case of transmission of one or two layers, it is possible to transmit a reference signal of each layer by using only a single code division multiplexing reference signal group 301. Therefore, the other code division multiplexing reference signal group 302 may be used for data transmission. The code division reference signal, corresponding to each layer, is transmitted by applying the same pre-coding applied to that layer. This makes it possible for a receiver to decode data without any information about a pre-coding applied in a transmitter.
FIG. 4 is a diagram illustrating a downlink transmission method based on multi-user multi-input multi-output in a conventional LTE-Advanced system.
FIG. 4 shows a way of transmitting data channels of multiple user equipment through the same resource by using a space division multiplexing technique at transmission using multiple antennas. In particular, the space division multiplexing technique achieves pre-coding data channels of multiple user equipment, with little interference with the space channels of the user equipment, followed by a transmission through the same resource. By using this technique, PDSCHs 404 to 407 of multiple user equipment can be transmitted through a single RB 402 in a PDSCH region 401. Here, PDSCHs 404 to 407 corresponding to respective layers are transmitted together with the reference signal groups 301 and 302 shown in FIG. 3. A decoding of PDSCHs 404 and 405 for layers 0 and 1 uses a channel value estimated from one code division multiplexing reference signal group 301 for the layers 0 and 1, and a decoding of PDSCHs 406 and 407 for layers 2 and 3 uses a channel value estimated from the other code division multiplexing reference signal group 302 for the layers 2 and 3.
However, the above-discussed wireless communication system based on multiple antennas may still confront a lack of control channel resources.